1. Field of the Invention
The present invention relates to color shift correcting techniques and, more particularly, to a color shift correcting apparatus and method for image forming apparatus to correct color shift in an output image of an image forming apparatus and a color shift correcting program and a recording medium storing a color shift correcting program.
2. Description of the Related Art
Conventionally, in an image forming apparatus for forming an image of a plurality of colors such as a color printer, a color image is formed by overlapping images of different colors such as, for example, magenta, cyan, yellow and black. Accordingly, when so-called color shift occurs due to an offset in positions of images of each color, a color of characters and lines may be changed or image irregularity (color irregularity) may occur, which results in deterioration of image quality. Thus, it is desirous to form a color image by matching positions of images of each color as much as possible.
Especially, in a tandem-type image forming apparatus having a photosensitive member and a write optical system for writing an image on the photosensitive member by a light beams or the like for each color, since color images are formed by different write optical systems and different photosensitive members, it is an important issue to eliminate color shift.
In order to correct color shift, a correction pattern for correcting color shift in an image is formed on a transfer belt so as to detect an amount of color shift of an image by reading the formed correction pattern by an optical sensor. Then, based on the result of the detection, a timing for writing an image on a photosensitive member by a light beam is adjusted or a mirror of an optical system provided in an apparatus is moved and adjusted so as to perform a color shift correction (for example, refer to Patent Documents 1, 2 and 3).
FIG. 1 is an illustration of an example of a color shift detection pattern. As shown in FIG. 1, an amount of color shift is small at a position where a sensor output voltage is low. FIG. 2 is a flowchart of a conventional example of a color shift correcting process. First, a color shirt detection pattern is printed on a belt (step S1). Then, a signal of a sensor (a signal corresponding to the color shift detection pattern) is read (step S2) so as to detect an amount of color shift (step S3).
Here, there is a detectable range of a maximum amount of color shift. If the present amount of color shift is within the detectable range, color shift can be detected normally. However, if an amount of color shift is abnormally large and exceeds the detectable range, or an amount of color shift does not exceed but cannot be detected normally, such a state is rendered to be a color shift amount detection abnormality. Additionally, if an amount of color shift exceeds a correctable range, such a state is rendered to be a correction amount abnormality.
If it is not a color shift amount detection abnormality (N of step S4), and if it is not a correction amount abnormality (N of step S5), a color shift correction is performed normally (step S6). On the other hand, if it is a color shift amount detection abnormality (Y of step S4), or if it is a correction amount abnormality (Y of step S5), an error process if performed conventionally (step S7).
Patent Document 1: Japanese Laid-Open Patent Application No. 6-253151
Patent Document 2: Japanese Laid-Open Patent Application No. 8-85234
Patent Document 3: Japanese Laid-Open Patent Application No. 8-305110
However, according to the image forming apparatuses of a conventional technique disclosed in the above mentioned Patent Documents 1-3, there is a detectable range in which an amount of color shift can be detected. Thus, if an amount of color shift is large and exceeds the detectable range, there is a case where an amount of color shift cannot be detected normally.
In such as case, a color shift correction cannot be performed since an amount of color shift to be corrected is unknown, and the color shift correcting process must be ended abnormally. At this time, the original color shift remains uncorrected. Thus, if the same procedure is repeated, the color shift amount cannot be detected again, and there is a problem in that the color shift correction cannot be performed.
Moreover, there is a correctable range in an amount of correction of color shift. Thus, even if the color shift amount in an image can be detected, there is a case where an amount of correction is too large due to a large amount of color shift and exceeds the correctable range. Also in such a case, there may be a case where an amount of correction is too large due to a large amount of color shift and exceeds the correctable range. In this case, the color shift correcting process must be ended abnormally as well as the above mentioned case, and there is a problem in that a color shift correction cannot be performed.